Increasing oil prices and national legislation demand the reduction of automotive carbon dioxide emissions and, therefore, force tire and rubber producers to provide “fuel efficient” tires. One general approach to obtain fuel efficient tires is to produce rubber formulations that have reduced hysteresis loss.
A major source of hysteresis in vulcanized elastomeric polymers is attributed to free polymer chain ends, i.e. that section of the elastomeric polymer chain between the last crosslink and the end of the polymer chain. This free end of the polymer does not participate in any efficient elastically recoverable process. As a result, any energy transmitted to this section of the polymer is lost. The dissipated energy leads to a pronounced hysteresis under dynamic deformation.
Another source of hysteresis in vulcanized elastomeric polymers is attributed to an insufficient distribution of filler particles in the vulcanized elastomeric polymer composition. The hysteresis loss of a crosslinked elastomeric polymer composition is related to its tan δ at 60° C. value (see ISO 4664-1:2005; Rubber, Vulcanized or thermoplastic; Determination of dynamic properties—part 1: General guidance). In general, vulcanized elastomeric polymer compositions having a relatively small tan δ at 60° C. are preferred for having a reduced hysteresis loss. In the final tire product, this translates into a lower rolling resistance and better fuel efficiency.
One generally accepted approach for reducing hysteresis loss lies in the reduction of the number of free chain ends of elastomeric polymers. Various techniques have been described in the literature, including the use of coupling agents such as tin tetrachloride, which may functionalize the polymer chain end and react with, for example, filler or unsaturated portions of the polymer (see U.S. Pat. Nos. 3,281,383, 3,244,664, 3,692,874, 3,978,103, 4,048,206, 4,474,908, 6,777,569, 3,078,254 and 4,616,069 and US 2005/0124740). The use of coupling agents as reactants for living polymers usually results in the formation of polymer blends comprising one fraction of linear or non-coupled polymers and one or more fractions comprising more than two polymer arms at the coupling point. Silicon tetrahalide is a typical example of silicone halide-based coupling agents.
WO 2007/047943 describes the use of a silane-sulfide modifier to produce a chain end-modified elastomeric polymer which is used in a vulcanized elastomeric polymer composition such as a tire tread. This document describes the reaction between a silane-sulfide compound and anionically-initiated living polymers to produce chain end-modified polymers which are subsequently blended and reacted with fillers, vulcanizing agents, accelerators or oil extenders to produce a vulcanized elastomeric polymer composition having low hysteresis loss. To further control polymer molecular weight and polymer properties, a coupling agent (“linking agent”) can be used as an optional component in the process. The modifier is then added before, after or during the addition of the coupling agent, and the modification reaction is preferably carried out after the addition of the coupling agent.
WO 2009/148932 describes the combined use of two modifier compounds which are sequentially reacted with a living anionic elastomeric polymer to produce a modified elastomeric polymer in order to reduce the tire-related rolling resistance and associated fuel consumption.
Despite the improvement in hysteresis loss, the teaching provided in WO 2007/047943 and WO 2009/148932 provides vulcanized articles having a wet skid resistance (“wet grip performance”) which is insufficient for certain applications.
Low vinyl bond polymers having a low proportion of 1,2-polybutadiene in the polybutadiene fraction of the butadiene copolymer or in polybutadiene homopolymer are used in tire treads and tire side wall compounds owing to their improved abrasion resistance. In contrast, their wet grip performance is unsatisfactory. The best polymers on the market in terms of their abrasion resistance are high cis-polybutadienes produced with a neodymium-based Ziegler-Natta catalyst and having a vinyl bond content of <2% and a glass transition temperature (Tg) in the order of about −105° C. As a result, low vinyl bond polymers have been used mainly as additives in polymer mixtures for improving abrasion resistance.
Wet grip and rolling resistance of a vulcanized polymer also depend strongly on its Tg, which again is influenced by the vinyl bond content in the vulcanized polymer, i.e. a low vinyl bond content is associated with a low Tg. For accomplishing beneficial properties of a vulcanized elastomeric polymer composition for use in tire treads, there is the desire to balance abrasion resistance and wet grip while simultaneously lowering both rolling resistance and heat build-up as much as possible.